Wireless Communication with GNU SDR

1
Wireless Communication with GNU SDR
2
A simplified wireless communication scheme
3
GNU Software Defined Radio

• Basically, the idea is to generate the baseband
  waveform in the computer and send it to the USRP.
  
• The USRP converts the digital waveform into
  analog waveform and send it out.
• The receiver does the reverse.

4
The code for a simple BPSK transmitter and
receiver

• http//www.cs.fsu.edu/zzhang/CIS5930_Spring_2009_
  files/tea1.py
• Basically, the signal processing functions are
  implemented in C as signal processing
  blocks. The blocks can be considered as
  equivalent to hardware chips with inputs and
  outputs, and a black box which process the inputs
  into outputs.
• The signal processing blocks are connected in
  Python.
• You will have a signal source and a signal sink.

5
The sender

• Signal Source. Basically a series of bytes
• self.bytes_src1 gr.vector_source_b(array1,
  True)
• Turn bytes into k-bit vectors
• self.bytes2chunks gr.packed_to_unpacked_bb(self.
  bits_per_symbol(), gr.GR_MSB_FIRST)
• Turn k-bits into symbols
• self.chunks2symbols gr.chunks_to_symbols_bc(psk.
  constellation2)

6
The sender

• The low pass filter
• self.rrc_filter gr.interp_fir_filter_ccf(self._s
  amples_per_symbol, self.rrc_taps)
• Then send it to the signal sink. Thats it!
• fg.connect(self.bytes_src1, self.bytes2chunks,
  self.chunks2symbols, self.rrc_filter, self.amp,
  self.u)
• For debugging, you can also connect the output of
  any signal processing block to a file sink, and
  play it

7
The Receiver

• First, there is signal source self.u (USRP)
• Then, pass it to amplifier, channel filter, then
  to rrc_filter, then to the gr.mpsk_receiver_cc,
  which turns the received baseband signal into
  bits (its actually symbols that can be
  immediately converted to bits)
• self.receivergr.mpsk_receiver_cc(arity, 0,
• 
  self._costas_alpha, self._costas_beta,
• fmin,
  fmax,
• 
  self._mu, self._gain_mu,
• omega,
  gain_omega,
• 
  _def_omega_relative_limit)

8
The baseband waveform

• Check the linux machine

9
Complex representations

• You have to get used to representing the received
  signal as complex numbers.
• That is, r(t) Re(t) jIm(t).
• Why?
• Because if you will multiply the r(t) with both
  cos(2\pi ft) and sin(2\pi ft), and both will be
  sent to a LPF. The one corresponding to cos(2\pi
  ft) is regarded as the real part and the one
  corresponding to sin(2\pi ft) is regarded as the
  imaginary part.

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Complex representations

• Suppose the sender sends I(t)cos(2 \pi ft). It
  can be considered as I(t)cos(2 \pi ft) 0 sin(2
  \pi ft).
• Suppose the phase difference between the sender
  and the receiver is \phi. So, the receiver will
  get I(t)cos(\phi) in one branch and
  I(t)sin(\phi) in the other branch (you can verify
  it). Its therefore natural to regard the two
  signals as the real and the imaginary component
  of signal I(t) ej\phi.

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Frequency representation

• You must also get used to the frequency
  representation of the time domain signal.
• Time domain cos(2\pi ft). To specify it, you
  have to specify its value at every time instant.
• Frequency domain a cosine wave with frequency
  f. Thats it.
• They are basically different languages talking
  about exactly the same thing.

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Frequency

• As mentioned last time, the received signal can
  be regarded as the summation of sine waves at
  various frequencies.
• F(w) \int_-\infty\infty f(t) e-jwt dt.
• f(t) \int_-\infty\infty F(w) ejwt dw.
• Example.
• f(t) cos(w_0 t). F(w) \infty (ww_0, -w_0),
  and 0 in all other values.
• http//www.analyzemath.com/trigonometry/trigonomet
  ric_formulas.html